The antibiotic sensitivity of 4095 Salmonella typhi strains isolated in Hungary from January, 1974 to June 1979, was tested. Twelve strains derived from one patient and seven chronic carriers were resistant to antibiotics due to R-plasmids. One of the S. typhi strains carried two R-plasmids and the change in the phage type of this strain was caused by one of the plasmids. R-plasmid of the same restrictive property was found in an Escherichia coli strain isolated from the faeces of the corresponding carrier. All of the R-plasmids examined were fi-, E. coli and Shigella flexneri phages were restricted by four R-plasmids, S. typhi phages were restricted by one. R-plasmids belonged to incompatibility groups I1, I2, W and H.